a. Field of the Invention
The invention is directed to a method for producing a line-shaped weakening in a flat material by local removal of material on one side of the flat material by means of controllable pulsed laser radiation. The flat material can be planar or three-dimensional.
b. Description of the Related Art
It is conventional for many applications to produce a line-shaped weakening of material in order to create a predetermined breaking line which can be broken, in case of need, by the application of force in order to separate the adjoining parts of material from one another or to form an opening. It is always advantageous when such predetermined breaking lines have a constant resistance to breaking along their length so that a severing can be effected with constant force. For various applications it is even required for technical reasons concerning safety that the breaking resistance be producible in a constant and reproducible manner. One such application, for example, is an airbag cover. For cosmetic reasons, it is also sometimes demanded that the predetermined breaking line be invisible to the naked eye of the vehicle passengers.
If the advantages of laser machining are to be used for producing such line-shaped weakened portions, it becomes difficult to meet the demands for a reproducible, constant breaking resistance. A reproducible, constant breaking resistance essentially requires that the residual wall thickness in the area of the line-shaped weakening be producible in a constant and reproducible manner. This can be achieved by uniform removal depth, provided the material has a constant thickness.
But even with homogeneous materials of constant thickness, it is unlikely that a defined residual wall thickness can be achieved by an uncontrolled machining laser. Fluctuations in radiation quality and radiation output lead to corresponding fluctuations in machining depth. A controlling of the laser as a function of the removal depth is required, which presupposes the sensing of the removal depth. The use of mechanical measuring means and, thus, a contacting sensing of the removal depth is ruled out by reason of the small cut gap widths. Nor can electrical or magnetic measurement processes be applied when machining electrically nonconducting materials.
A method for material removal, especially of metal workpieces, with laser radiation is described in DE 39 43 523 C2. According to the invention, the effective laser radiation intensity is regulated by switching the laser on and off depending on the detected heat radiation, which appears unsuitable for higher machining speeds. In order to improve accuracy, the removal depth is measured and taken into account for correcting limiting values. It is mentioned in the description of DE 39 43 527 C2 that the measurement of the removal depth is effected, for example, with an optical sensor working on the principle of triangulation. However, the applicability of this measurement principle is limited to cuts of vertical configuration, cut gap widths greater than the cross section of the measurement beam, and a linear cutting path. Moreover, like other measurement principles which detect removal depth, this measurement principle is not suited to indicate the remaining residual wall thickness if the material thickness is not constant.
The influence of a material thickness which is not constant on the breaking resistance of the predetermined breaking line can only be ruled out by a removal which is regulated depending on the detected residual wall thickness. A solution according to DE 43 20 341 A1 is apparently offered in this connection. A method is disclosed for removal of cover layers of glass building components by laser radiation in which the removal process is regulated in dependence on transmittance values. In this case, the aim is to remove the cover layer of a glass building component in a purposeful and defined manner such that a residual cover layer with a predetermined layer thickness distribution remains behind on the glass building component. The layer thickness remaining on the glass building component can be indicated by way of measuring the transmittance at the machining location. Accordingly, this measuring method is especially suited for removal of uneven layer thicknesses to a given residual layer thickness without the need to determine the topology of the uneven cover thickness beforehand.
In order to determine the residual layer thickness, the intensity of a transmitted measurement beam is measured at the machining location after every machining laser pulse and the measured value is fed to a process computer. By combining individual measurement values, various evaluation criteria can be determined, such as removal efficiency and relative transmission. The measurement value is compared, as an individual value, with a threshold value and when this threshold is exceeded serves as a switch-off criterion for terminating the removal process.
As has been shown in practical tests, the solution according to DE 43 20 341 cannot be successfully applied for producing a line-shaped weakening of desired quality. In contrast to the described removal of cover layers on glass building components where the removal is effected in a planar manner so that occurring combustion residue and evaporation gases are immediately distributed and evaporated or volatilized in a planar manner, these occurring gases and residues remain concentrated at the removal site for longer periods of time when removal is effected in a line-shaped manner, especially when the removal width is substantially smaller than the removal depth, and falsify the transmittance values due to their absorption.